Method of manufacturing magnetic recording medium glass substrate

ABSTRACT

Provided is a magnetic recording medium glass substrate manufacturing method capable of manufacturing a magnetic recording medium glass substrate having high surface smoothness and low surface waviness with productivity. In a primary lapping process and a secondary lapping process of the manufacturing method, diamond pads ( 20 A and  20 B) in which diamond abrasive grains are fixed by a binder are used. A plurality of convex portions ( 21 ) with a flat top are arranged in a tile shape on a lapping surface ( 20   a ) of each of the diamond pads ( 20 A and  20 B). In the diamond pad ( 20 A) used in the primary lapping process, the average diameter of the diamond abrasive grains is in the range of 4 μm to 12 μm and the content of the diamond abrasive grains in the convex portion ( 20 A) is in the range of 5 vol % to 70 vol %. In the diamond pad ( 20 B) used in the secondary lapping process, the average diameter of the diamond abrasive grains is in the range of 1 μm to 5 μm and the content of the diamond abrasive grains in the convex portion ( 20 B) is in the range of 5 vol % to 80 vol %.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a magneticrecording medium glass substrate.

Priority is claimed on Japanese Patent Application No. 2009-257112,filed Nov. 10, 2009, and Japanese Patent Application No. 2010-182326,filed Aug. 17, 2010, the content of which is incorporated herein byreference.

BACKGROUND ART

The recording density of a magnetic recording medium used in a hard diskdrive (HDD) has been significantly improved. In particular, with theintroduction of an MR head or a PRML technique, the surface recordingdensity of the magnetic recording medium has significantly increased. Inrecent years, for example, a GMR head or a TMR head has been introducedand the surface recording density has increased at a rate of 1.5 timesper year. However, there is a demand for an improvement in recordingdensity.

With the increase in the recording density of the magnetic recordingmedium, there is an increasing demand for a substrate for the magneticrecording medium. An aluminum alloy substrate and a glass substrate havebeen used as the substrate for the magnetic recording medium. Of the twokinds of substrates, in general, the hardness, surface smoothness,rigidity, and impact resistance of the glass substrate are better thanthose of the aluminum alloy substrate. Therefore, a magnetic recordingmedium glass substrate capable of improving the recording density hasreceived increased attention.

When the magnetic recording medium glass substrate is manufactured, alarge glass plate is cut into disk-shaped glass substrates, or adisk-shaped glass substrate is directly formed from molten glass bypress-molding using a mold. Then, a lapping (grinding) process and apolishing process are performed on the main surface and end surface ofthe obtained glass substrate.

In a method of manufacturing a magnetic recording medium glass substrateaccording to the related art, a primary lapping process (grinding), asecondary lapping process (grinding), a primary polishing process(polishing), and a secondary polishing process (polishing) are performedon the main surface of the glass substrate in this order. Then, alapping process and a polishing process for the inner and outercircumferential end surfaces of the glass substrate are performedbetween the processes.

Regarding the related art of the invention, for example, the followingPTL 1 is known. PTL 1 discloses a technique which performs a primarylapping process using a diamond pellet, such as a resin, metal, orvitrified grinding wheel, and a secondary lapping process using adiamond pad to process a substrate in a short time without incurringdefects, such as low surface smoothness, scratches, grinding marks, orsuction marks.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 4049510

SUMMARY OF INVENTION Technical Problem

However, in recent years, with a reduction in the floating height of amagnetic head, there is a demand for a magnetic recording medium glasssubstrate with a surface waviness or a surface roughness lower than thatin the related art. The inventors found that the grinding allowance ofone surface was in the range of 100 μm to 300 μm in the primary lappingprocess, and when the glass substrate was damaged in the primary lappingprocess, processing strain occurred in the glass substrate, which causedlong-period waviness on the surface of the end-product magneticrecording medium.

The invention has been made in view of the above-mentioned problems, andan object of the invention is to provide a magnetic recording mediumglass substrate manufacturing method capable of manufacturing a magneticrecording medium glass substrate having high surface smoothness and lowsurface waviness with high productivity.

Solution to Problem

The invention provides the following means.

According to a first aspect of the invention, there is provided a methodof manufacturing a magnetic recording medium glass substrate. The methodincludes a step of performing a primary lapping process on a surface ofthe glass substrate except for at least an end surface and a step ofperforming a secondary lapping process on the surface of the glasssubstrate. The primary lapping process and the secondary lapping processuse diamond pads in which diamond abrasive grains are fixed by a binder.A plurality of convex portions with a flat top are arranged in a tileshape on a lapping surface of the diamond pad. In the diamond pad usedin the primary lapping process, the average diameter of the diamondabrasive grains is in the range of 4 μm to 12 μm and the content of thediamond abrasive grains in the convex portion is in the range of 5 vol %to 70 vol %. In the diamond pad used in the secondary lapping process,the average diameter of the diamond abrasive grains is in the range of 1μm to 5 μm and the content of the diamond abrasive grains in the convexportion is in the range of 5 vol % to 80 vol %.

According to a second aspect of the invention, in the method ofmanufacturing a magnetic recording medium glass substrate according tothe first aspect, in the diamond pads used in the primary lappingprocess and the secondary lapping process, the convex portion may haveoutside dimensions including a size of 1.5 mm to 5 mm×1.5 mm to 5 mm anda height of 0.2 mm to 3 mm, and a gap between adjacent convex portionsmay be in the range of 0.5 mm to 3 mm.

Advantageous Effects of Invention

As described above, in the invention, the diamond pads are used in theprimary lapping process and the secondary lapping process, and thediamond pads used in the primary lapping process and the secondarylapping process are optimized. Therefore, it is possible to manufacturea magnetic recording medium glass substrate having high surfacesmoothness and low surface waviness with high productivity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a process of manufacturing a magneticrecording medium glass substrate according to the invention and is aperspective view illustrating a main surface lapping process.

FIG. 2A is an enlarged plan view illustrating a pad surface of a diamondpad used in the main surface lapping process.

FIG. 2B is an enlarged cross-sectional view illustrating the diamond padused in the main surface lapping process taken along the line A-A′.

FIG. 3 is a diagram illustrating the process of manufacturing themagnetic recording medium glass substrate according to the invention andis a perspective view illustrating an inner/outer circumferential endsurface lapping process.

FIG. 4 is a diagram illustrating the process of manufacturing themagnetic recording medium glass substrate according to the invention andis a perspective view illustrating an inner circumferential end surfacepolishing process.

FIG. 5 is a diagram illustrating the process of manufacturing themagnetic recording medium glass substrate according to the invention andis a perspective view illustrating an outer circumferential end surfacepolishing process.

FIG. 6 is a diagram illustrating the process of manufacturing themagnetic recording medium glass substrate according to the invention andis a perspective view illustrating a main surface polishing process.

FIG. 7 is a perspective view illustrating another example of thestructure of a lapping machine or a polishing machine used in theinvention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a method of manufacturing a magnetic recording medium glasssubstrate according to the invention will be described in detail withreference to the accompanying drawings.

The magnetic recording medium glass substrate manufactured by theinvention is a disk-shaped glass substrate having a central hole. Amagnetic recording medium is obtained by sequentially forming, forexample, a magnetic layer, a protective layer, and a lubrication film onthe surface of the glass substrate. In a magnetic recording/reproducingdevice (HDD), the center of the magnetic recording medium is attached toa rotating shaft of a spindle motor and a magnetic head writes or readsinformation to or from the magnetic recording medium while floating andtraveling on the surface of the magnetic recording medium rotated by thespindle motor. The magnetic recording medium glass substrate may be madeof, for example, SiO₂—Al₂O₃—R₂O-based chemical strengthening glass (Rindicates at least one kind of element selected from alkali metalelements), SiO₂—Al₂O₃—Li₂O-based glass ceramics, orSiO₂—Al₂O₃—MgO—TiO₂-based glass ceramics. Among them, in particular, forexample, SiO₂—Al₂O₃—MgO—CaO—Li₂O—Na₂O—ZrO₂—Y₂O₃—TiO₂—As₂O₃-basedchemical strengthening glass, SiO₂—Al₂O₃—Li₂O—Na₂O—ZrO₂—As₂O₃-basedchemical strengthening glass,SiO₂—Al₂O₃—MgO—ZnO—Li₂O—P₂O₅—ZrO₂—K₂O—Sb₂O₃-based glass ceramics,SiO₂—Al₂O₃—MgO—CaO—BaO—TiO₂—P₂O₅—As₂O₃-based glass ceramics, andSiO₂—Al₂O₃—MgO—CaO—SrO—BaO—TiO₂—ZrO₂—Bi₂O₃—Sb₂O₃-based glass ceramicsmay be appropriately used. In addition, for example, the magneticrecording medium glass substrate is made of lithium disilicate, aSiO₂-based crystal (for example, quartz, cristobalite, tridymite),cordierite, enstatite, aluminum magnesium titanate, spinel-based crystal([Mg and/or Zn]Al₂O₄, [Mg and/or Zn]₂TiO₄, and a solid fluid between thetwo crystals), forsterite, spodumene, and glass ceramics having, forexample, solid fluids of these crystals as crystal phases.

When the magnetic recording medium glass substrate is manufactured,first, a large flat glass plate is cut into glass substrates, or a glasssubstrate is directly formed from molten glass by press-molding using amold. In this way, the disk-shaped glass substrate having a central holeis obtained.

Then, a lapping (grinding) process and a polishing process are performedon the surface (the main surface) of the obtained glass substrate exceptfor an end surface. In addition, a process of performing lapping andpolishing on the inner and outer circumferential end surfaces of theglass substrate is preformed between the two processes. In theinvention, a chamfering process may be performed on the inner and outercircumferential end surfaces of the glass substrate, simultaneously withthe lapping process.

Specifically, in this embodiment, a primary main surface lappingprocess, an inner/outer circumferential end surface lapping process, aninner circumferential end surface polishing process, a secondary mainsurface lapping process, an outer circumferential end surface polishingprocess, a primary main surface polishing process, and a secondary mainsurface polishing process are performed in this order.

Among them, in the primary main surface lapping process, the primarylapping process is performed on two main surfaces (finally, a recordingsurface of the magnetic recording medium) of a glass substrate W using alapping machine 10 shown in FIG. 1. That is, the lapping machine 10includes a pair of upper and lower plates 11 and 12. A plurality ofglass substrates W are interposed between the plates 11 and 12 which arerotated in the opposite direction. Then, the two main surfaces of eachglass substrate W are polished by grinding pads which are provided onthe plates 11 and 12.

As shown in FIGS. 2A and 2B, the grinding pad used in the primarylapping process is a diamond pad 20A in which diamond abrasive grainsare fixed by a binder (bond). A plurality of convex portions 21 with aflat top are arranged in a tile shape on a lapping surface 20 a of thediamond pad 20A. In addition, the diamond pad 20A is formed by arranginga plurality of convex portions 21 in which the diamond abrasive grainsare fixed by the binder on the surface of a base material 22.

In the diamond pad 20A used in the primary lapping process, it ispreferable that the convex portion 21 have the following outsidedimensions S: a size of 1.5 mm to 5 mm×1.5 mm to 5 mm; a height T of 0.2mm to 3 mm; and a gap G of 0.5 mm to 3 mm between adjacent convexportions 21. In the invention, when the diamond pad 20A satisfying theabove-mentioned ranges is used, it is possible to uniformly spread, forexample, a coolant or a grinding liquid and smoothly discharge grindingswarf between the convex portions 21 of the lapping surface 20 a.

In the diamond pad 20A used in the primary lapping process, the averagediameter of the diamond abrasive grains is in the range of 4 μm to 12 μmand preferably in the range of 6 μm to 12 μm, and the content of thediamond abrasive grains in the convex portion 21 is preferably in therange of 5 vol % to 70 vol %, more preferably in the range of 5 vol % to40 vol %, and most preferably in the range of 20 vol % to 30 vol %. Whenthe diameter and content of the diamond abrasive grains are less thanthe above-mentioned ranges, the processing time increases, which resultsin an increase in costs. On the other hand, when the diameter andcontent of the diamond abrasive grains are more than the above-mentionedranges, it is difficult to obtain a desired surface roughness. Thebinder of the diamond pad 20A may be made of a resin, such as apolyurethane-based resin, a phenol-based resin, a melamine-based resin,or an acryl-based resin.

In the inner/outer circumferential end surface lapping process, alapping process is performed on the inner circumferential end surface ofthe central hole of the glass substrate W and the outer circumferentialend surface of the glass substrate W using a lapping machine 30 shown inFIG. 3. That is, the lapping machine 30 includes an innercircumferential whetstone 31 and an outer circumferential whetstone 32.A laminate X obtained by laminating a plurality of glass substrates Wwith a spacer S therebetween and the central holes aligned with eachother is interposed between the inner circumferential whetstone 31 whichis inserted into the central holes of the glass substrates W and theouter circumferential whetstone 32 which is arranged on the outercircumferential sides of the glass substrates W in the diametricdirection of each glass substrate W while being rotated on its own axis,and the inner circumferential whetstone 31 and the outer circumferentialwhetstone 32 are rotated in a direction opposite to the rotationaldirection of the laminate X. Then, the inner circumferential end surfaceof each glass substrate W is ground by the inner circumferentialwhetstone 31. At the same time, the outer circumferential end surface ofeach glass substrate W is ground by the outer circumferential whetstone32.

The surface of each of the inner circumferential whetstone 31 and theouter circumferential whetstone 32 has a corrugated shape in whichconvex portions and concave portions are alternately arranged in theaxial direction. Therefore, the inner circumferential whetstone 31 andthe outer circumferential whetstone 32 can grind the innercircumferential end surface and the outer circumferential end surface ofeach glass substrate W and chamfer edge portions (chamfered surfaces)between the two main surfaces and the inner and outer circumferentialend surfaces of each glass substrate W. In this embodiment, the lappingprocess is performed on the inner and outer circumferential end surfacesof the glass substrate W in one stage, but the invention is not limitedthereto. The lapping process may be performed in two stages (primary andsecondary lapping processes).

In the inner circumferential whetstone 31 and the outer circumferentialwhetstone 32, the diamond abrasive grains are fixed by a binder. Thebinder may be made of a metal material, such as copper, copper alloy,cobalt, tungsten carbide, or nickel. It is preferable that the averagediameter of the diamond abrasive grains in the inner circumferentialwhetstone 31 and the outer circumferential whetstone 32 be equal to orgreater than 4 μm and equal to or less than 12 μm. The content of thediamond abrasive grains in the inner circumferential whetstone 31 andthe outer circumferential whetstone 32 is preferably in the range of 5vol % to 95 vol % and more preferably in the range of 20 vol % to 90 vol%. When the diameter and content of the diamond abrasive grains are lessthan the above-mentioned ranges, the processing time increases, whichresults in an increase in costs. On the other hand, when the diameterand content of the diamond abrasive grains are more than theabove-mentioned ranges, it is difficult to obtain a desired surfaceroughness.

In the inner circumferential end surface polishing process, a polishingprocess is performed on the inner circumferential end surface of thecentral hole of each glass substrate W using a polishing machine 40shown in FIG. 4. That is, the polishing machine 40 includes an innercircumference polishing brush 41. The laminate X is rotated on its ownaxis and the inner circumference polishing brush 41 inserted into thecentral hole of each glass substrate W is moved in the verticaldirection while being rotated in a direction opposite to the rotationaldirection of the glass substrate W. In this case, a polishing liquid isdropped onto the inner circumference polishing brush 41. Then, the innercircumferential end surface of each glass substrate W is polished by theinner circumference polishing brush 41. At the same time, the edgeportion (chamfered surface) of the inner circumferential end surfacechamfered by the inner/outer circumferential end surface lapping processis also polished. For example, slurry obtained by dispersing ceriumoxide abrasive grains in water may be used as the polishing liquid.

In the secondary main surface lapping process, similarly to the primarymain surface lapping process, a secondary lapping process is performedon two main surfaces of the glass substrate W using the lapping machine10 shown in FIG. 1. That is, a plurality of glass substrates W areinterposed between a pair of upper and lower plates 11 and 12 which arerotated in the opposite direction and the two main surfaces of eachglass substrate W are ground by the grinding pads which are provided onthe plates 11 and 12.

Similarly to the grinding pad 20A shown in FIGS. 2A and 2B, the grindingpad used in the secondary lapping process is a diamond pad 20B in whichdiamond abrasive grains are fixed by a binder (bond). A plurality ofconvex portions 21 with a flat top are arranged in a tile shape on alapping surface 20 a of the diamond pad 20B. In addition, the diamondpad 20B is formed by arranging a plurality of convex portions 21 inwhich the diamond abrasive grains are fixed by the binder on the surfaceof a base material 22.

Similarly to the diamond pad 20A shown in FIGS. 2A and 2B, in thediamond pad 20B used in the second lapping process, it is preferablethat the convex portion 21 have the following outside dimensions S: asize of 1.5 mm to 5 mm×1.5 mm to 5 mm; a height T of 0.2 mm to 3 mm; anda gap G of 0.5 mm to 3 mm between adjacent convex portions 21. In theinvention, when the diamond pad 20B satisfying the above-mentionedranges is used, it is possible to uniformly spread, for example, acoolant or a grinding liquid and smoothly discharge grinding swarfbetween the convex portions 21 of the lapping surface 20 a.

In the diamond pad 20B used in the secondary lapping process, theaverage diameter of the diamond abrasive grains is in the range of 1 μmto 5 μm and preferably in the range of 2 μm to 5 μm and the content ofthe diamond abrasive grains in the convex portion 21 is preferably inthe range of 5 vol % to 80 vol % and more preferably in the range of 50vol % to 70 vol %. When the diameter and content of the diamond abrasivegrains are less than the above-mentioned ranges, the processing timeincreases, which results in an increase in costs. On the other hand,when the diameter and content of the diamond abrasive grains are morethan the above-mentioned ranges, it is difficult to obtain a desiredsurface roughness. The binder of the diamond pad 20B may be made of aresin, such as a polyurethane-based resin, a phenol-based resin, amelamine-based resin, or an acryl-based resin.

In the outer circumferential end surface polishing process, a polishingprocess is performed on the outer circumferential end surface of theglass substrate W using a polishing machine 50 shown in FIG. 5. That is,the polishing machine 50 includes a rotating shaft 51 and an outercircumference polishing brush 52. A laminate X obtained by laminating aplurality of glass substrates W with a spacer S therebetween and thecentral holes aligned with each other is rotated on its own axis by therotating shaft 51 which is inserted into the central hole of each glasssubstrate W, and the outer circumference polishing brush 52 contactedwith the outer circumferential end surface of each glass substrate W ismoved in the vertical direction while being rotated in a directionopposite to the rotational direction of the laminate X. In this case, apolishing liquid is dropped onto the outer circumference polishing brush52. Then, the outer circumferential end surface of each glass substrateW is polished by the outer circumference polishing brush 52. At the sametime, the edge portion (chamfered surface) of the outer circumferentialend surface chamfered in the inner/outer circumferential end surfacelapping process is also polished. For example, slurry obtained bydispersing cerium oxide abrasive grains in water may be used as thepolishing liquid.

In the primary main surface polishing process, a primary polishingprocess is performed on two main surfaces of each glass substrate Wusing a polishing machine 60 shown in FIG. 6. That is, the polishingmachine 60 includes a pair of upper and lower plates 61 and 62. Aplurality of glass substrates W are interposed between the plates 61 and62 which are rotated in the opposite direction, and two main surfaces ofeach glass substrate W are polished by grinding pads which are providedon the plates 61 and 62.

The polishing pad used in the primary polishing process is, for example,a hard abrasive cloth made of urethane. When two main surfaces of theglass substrate W are polished (ground) by the polishing pad, apolishing liquid is dropped onto the glass substrate W. For example,slurry obtained by dispersing cerium oxide abrasive grains in water maybe used as the polishing liquid.

Similarly to the primary main surface polishing process, in thesecondary main surface polishing process, a secondary polishing processis performed on two main surfaces of each glass substrate W using thepolishing machine 60 shown in FIG. 6. That is, a plurality of glasssubstrates W are interposed between the plates 61 and 62 which arerotated in the opposite direction, and two main surfaces of each glasssubstrate W are polished by the grinding pads which are provided on theplates 61 and 62.

The polishing pad used in the secondary polishing process is, forexample, a soft abrasive cloth, such as suede. When two main surfaces ofthe glass substrate W are polished by the polishing pad, a polishingliquid is dropped onto the glass substrate W. For example, slurryobtained by dispersing cerium oxide abrasive grains or colloidal silicain a solvent, such as water, may be used as the polishing liquid.

Then, the glass substrate W subjected to the lapping process and thepolishing process is sent to a final cleaning process and a testprocess. In the final cleaning process, the glass substrate W is cleanedby, for example, a chemical cleaning method using a detergent (chemical)and ultrasonic wave and the polishing agent used in the above-mentionedprocesses is removed. In the test process, for example, an opticaltester using a laser is used to examine whether the surface (the mainsurface, the end surface, and the chamfered surface) of the glasssubstrate W is scratched or twisted.

In the method of manufacturing the magnetic recording medium glasssubstrate according to the invention, in the primary lapping process andthe secondary lapping process, the use of the diamond pads 20A and 20Bshown in FIGS. 2A and 2B makes it possible to smooth the surface of theglass substrate W except for the end surface in a short time whilesmoothly discharging grinding swarf between the convex portions 21 ofthe lapping surface 20 a. In addition, it is possible to reduce theprocessing time in the subsequent primary main surface polishing processand secondary main surface polishing process. Therefore, according tothe invention, it is possible to manufacture a magnetic recording mediumglass substrate having high surface smoothness and low surface wavinesswith high productivity.

In the invention, grinding liquids on the market may be used in theprimary lapping process and the secondary lapping process. The grindingliquids are mainly classified into aqueous grinding liquids and oilgrinding liquids. The aqueous grinding liquid includes, for example,pure water, an appropriate amount of alcohol, polyethyleneglycol servingas a viscosity modifier, amine, and a surface-active agent. The oilgrinding liquid includes oil and a stearic acid serving as anextreme-pressure additive. For example, aqueous Sabrelube 9016(manufactured by Chemetall) may be used as the grinding liquid on themarket.

In the invention, a polishing aid or an anticorrosive may be added tothe grinding liquids used in the primary lapping process and thesecondary lapping process, and the polishing liquids used in the primarypolishing process and the secondary polishing process.

Specifically, the polishing aid includes organic polymers having atleast a sulfonic acid group or a carboxylic acid group. Among them, itis preferable to use an organic polymer including sodium sulfonate orsodium carboxylate and having an average molecular weight of 4000 to10000. In this way, it is possible to further improve the smoothness ofthe surface of the glass substrate W in the above-mentioned processes.

Examples of the organic polymer including sodium sulfonate or sodiumcarboxylate includes GEROPON SC/213 (trade name/Rhodia), GEROPON T/36(trade name/Rhodia), GEROPON TA/10 (trade name/Rhodia), GEROPON TA/72(trade name/Rhodia), Newcalgen WG-5 (trade name/Takemoto Oil & Fat Co.,Ltd.), Agrisol G-200 (trade name/Kao Corporation), Demol EP powder(trade name/Kao Corporation), Demol RNL (trade name/Kao Corporation),Isoban 600-SF35 (trade name/Kuraray Co., Ltd.), Polystar OM (tradename/NOF Corporation), Sokalan CP9 (trade name/BASF Japan Ltd.), SokalanPA-15 (trade name/BASF Japan Ltd.), Toxanon GR-31A (trade name/SanyoChemical Industries, Ltd.), Solpol 7248 (trade name/Toho ChemicalIndustry Co., Ltd.), Sharoll AN-103P (trade name/Dai-Ichi Kogyo SeiyakuCo., Ltd.), Aron T-40 (trade name/Toagosei Co., Ltd.), Panakayaku CP(trade name/Nippon Kayaku Co., Ltd.), and Disrol H12C (trade name/NipponNyukazai Co., Ltd.). Among these, in particular, it is desirable to useDemol RNL (trade name/Kao Corporation) or Polystar OM (trade name/by NOFCorporation) as the polishing aid.

In the magnetic recording medium which is manufactured using the glasssubstrate W, in general, the magnetic layer includes a corrosion-pronematerial, such as Co, Ni, or Fe. Therefore, an anticorrosive is added tothe grinding liquid or the polishing liquid to prevent the corrosion ofthe magnetic layer. In this way, it is possible to obtain a magneticrecording medium with good electromagnetic conversion properties.

It is preferable that benzotriazole or derivatives thereof be used asthe anticorrosive. For example, a compound obtained by replacing one ortwo or more hydrogen atoms of benzotriazole with a carboxyl group, amethyl group, an amino group, or a hydroxyl group may be used as thebenzotriazole derivative. Examples of the benzotriazole derivativeinclude 4-carboxybenzotriazole or its salt, 7-carboxybenzotriazole orits salt, benzotriazole butyl ester, 1-hydroxymethylbenzotriazole, and1-hydroxybenzotriazole. The amount of anticorrosive added is preferablyequal to or less than 1 mass % and more preferably in the range of 0.001mass % to 0.1 mass % with respect to the total amount when the diamondslurry is used.

The invention is not necessarily limited to the above-describedembodiment, but various modifications and changes of the invention canbe made without departing from the scope and spirit of the invention.

For example, as shown in FIG. 7, the lapping machine 10 used in theprimary main surface lapping process and the secondary main surfacelapping process and the polishing machine 50 used in the primary mainsurface polishing process and the secondary main surface polishingprocess may have the following structure: each of the lapping machine 10and the polishing machine 50 includes a pair of upper and lower plates71 and 72 and a plurality of carriers 73 which are provided on onesurface of the lower plate 71 facing the upper plate 72; glasssubstrates'(not shown) are set in a plurality of openings 74 (in thisembodiment, 35 openings) provided in each of the carriers 73; and twomain surfaces of each of the plurality of glass substrates are ground orpolished by the grinding pads or the polishing pads provided on thelower plate 71 and the upper plate 72.

Specifically, the rotating shafts 71 a and 72 a provided at the centersof the lower plate 71 and the upper plate 72 are rotated by a motor (notshown) such that the lower plate 71 and the upper plate 72 can berotated in the opposite direction with the central axes aligned witheach other. In addition, a concave portion 75 for arranging a pluralityof carriers 73 (in this embodiment, five carriers) is provided in thesurface of the lower plate 71 facing the upper plate 72.

The plurality of carriers 73 are obtained by forming a reinforced epoxyresin having aramid fiber or glass fiber mixed therewith in a diskshape. The plurality of carriers 73 are arranged around the rotatingshaft 71 a in the concave portion 75. A planetary gear unit 76 isprovided over the entire outer circumferential portion of each carrier73. A sun gear unit 77 which is engaged with the planetary gear 76 ofeach carrier 73 and is rotated together with the rotating shaft 71 a isprovided in the inner circumferential portion of the concave portion 75,and a fixed gear unit 78 which is engaged with the planetary gear unit76 of each carrier 73 is provided in the outer circumferential portionof the concave portion 75.

In this way, when the sun gear unit 77 is rotated together with therotating shaft 71 a, the plurality of carriers 73 make a so-calledsun-and-planet motion in which they are rotated on their own axes in adirection opposite to the rotational direction of the rotating shaft 71a while being rotated around the rotating shaft 71 a in the concaveportion 75 in the same direction as that in which the rotating shaft 71a is rotated, by the engagement between the sun gear unit 77 and thefixed gear unit 78, and the planetary gear units 76.

Therefore, the lapping machine 10 and the polishing machine 50 havingthe above-mentioned structure can grind or polish two main surfaces ofeach of the plurality of glass substrates held in the openings 75 ofeach carrier 73 with the grinding pads or the polishing pads provided onthe lower plate 71 and the upper plate 72 while moving the plurality ofglass substrates as in the sun and planet system. In this structure, itis possible to rapidly grind or polish the glass substrates with highaccuracy.

EXAMPLES

Next, the effect of the invention will be clearly described withreference to examples. The invention is not limited to the followingexamples, but various modifications and changes to the invention can bemade without departing from the scope and spirit of the invention.

Example 1

In Example 1, first, a glass substrate (TS-10SX manufactured by Ohara,Inc.) with an outside diameter of 48 mm, a central hole diameter of 12mm, and a thickness of 0.560 mm was used.

The primary main surface lapping process, the inner/outercircumferential end surface lapping process, the inner circumferentialend surface polishing process, the secondary main surface lappingprocess, the outer circumferential end surface polishing process, theprimary main surface polishing process, and the secondary main surfacepolishing process were performed on the glass substrate in this order.

Specifically, in the primary main surface lapping process, a pluralityof glass substrates were interposed between a pair of upper and lowerplates, which were rotated in the opposite direction, of the lappingmachine and the two main surfaces of each of the glass substrates wereground by the grinding pads which are provided on the plates. In thiscase, a diamond pad (Trizact (trade name) manufactured by Sumitomo 3MLimited) was used as the grinding pad in the primary lapping process. Inthe diamond pad, the outside dimensions of a convex portion were asfollows: the convex portion had a size of 2.6 mm×2.6 mm and a height of2 mm; the gap between the convex portions was 1 mm; the average diameterof diamond abrasive grains was 9 μm; the content of the diamond abrasivegrains in the convex portion was about 20 vol %; and an acryl-basedresin was used as a binder. A 4-way type double-sided polishing machine(16B manufactured by Hamai Co., Ltd.) was used as the lapping machineand grinding was performed under the following conditions: the number ofrotations of the plate: 25 rpm; pressing pressure: 120 g/cm²; and theprocessing time: 15 minutes. The grinding liquid obtained by dilutingSabrelube 9016 (manufactured by Chemetall) 1 to 10 with water was usedand the amount of grinding of one surface of each glass substrate wasabout 100 μm.

In the inner/outer circumferential end surface lapping process, alapping machine including an inner circumferential whetstone and anouter circumferential whetstone was used. A laminate obtained bylaminating a plurality of glass substrates with a spacer therebetweenand the central holes aligned with each other was interposed between theinner circumferential whetstone which was inserted into the centralholes of the glass substrates and the outer circumferential whetstonewhich was arranged on the outer circumferential sides of the glasssubstrates in the diametric direction of each glass substrate whilebeing rotated on its own axis. The inner circumferential whetstone andthe outer circumferential whetstone were rotated in a direction oppositeto the rotational direction of the laminate. In this way, the inner andouter circumferential end surfaces of each glass substrate wererespectively ground by the inner circumferential whetstone and the outercircumferential whetstone at the same time. In this case, as the innercircumferential whetstone and the outer circumferential whetstone, awhetstone was used in which the content of diamond abrasive grains withan average grain diameter of 10 μm was 80 vol % and a copper alloy wasused as the binder. Grinding was performed under the followingconditions: the number of rotations of the inner circumferentialwhetstone: 1200 rpm; the number of rotations of the outercircumferential whetstone: 600 rpm; and the processing time: 30 seconds.

In the inner circumferential end surface polishing process, a polishingmachine including an inner circumference polishing brush was used. Thelaminate was rotated on its own axis, and the inner circumferencepolishing brush inserted into the central hole of each glass substratewas moved in the vertical direction while was being rotated in adirection opposite to the rotational direction of the glass substrate.During the operation, a polishing liquid was dropped onto the innercircumference polishing brush. In this way, the inner circumferentialend surface of each glass substrate was polished. In this case, a nylonbrush was used as the inner circumference polishing brush and ceriaslurry was used as the polishing liquid. Polishing was performed underthe following conditions: the number of rotations of the innercircumference polishing brush: 300 rpm and the processing time: 10minutes.

In the secondary main surface lapping process, a lapping machineincluding a pair of upper and lower plates was used. A plurality ofglass substrates were interposed between the plates which were rotatedin the opposite direction and two main surfaces of each of the glasssubstrates were ground by the grinding pads which are provided on theplates. In this case, a diamond pad (Trizact (trade name) manufacturedby Sumitomo 3M Limited) was used as the grinding pad in the secondarylapping process. In the diamond pad, the outside dimensions of a convexportion were as follows: the convex portion had a size of 2.6 mm×2.6 mmand a height of 2 mm; the gap between the convex portions was 1 mm; theaverage diameter of diamond abrasive grains was 4 μm; the content of thediamond abrasive grains in the convex portion was about 50 vol %; and anacryl-based resin was used as a binder. A 4-way type double-sidedpolishing machine (16B manufactured by Hamai Co., Ltd.) was used as thelapping machine and grinding was performed under the followingconditions: the number of rotations of the plate: 25 rpm; pressingpressure: 120 g/cm²; and the processing time: 10 minutes. The grindingliquid obtained by diluting Sabrelube 9016 (manufactured by Chemetall) 1to 10 with water was used and the amount of grinding of one surface ofeach glass substrate was about 30 μm.

In the outer circumferential end surface polishing process, a polishingmachine including an outer circumference polishing brush was used. Alaminate obtained by laminating a plurality of glass substrates with aspacer therebetween and the central holes aligned with each other wasrotated on its own axis by a rotating shaft which was inserted into thecentral hole of each glass substrate, and the outer circumferencepolishing brush contacted with the outer circumferential end surface ofeach glass substrate was moved in the vertical direction while beingrotated in a direction opposite to the rotational direction of thelaminate. During the operation, a polishing liquid was dropped onto theouter circumference polishing brush. In this way, the outercircumferential end surface of each glass substrate was polished. Inthis case, a nylon brush was used as the outer circumference polishingbrush and ceria slurry was used as the polishing liquid. The polishingwas performed under the following conditions: the number of rotations ofthe outer circumference polishing brush: 300 rpm; and the processingtime: 10 minutes.

In the primary main surface polishing process, a polishing machineincluding a pair of upper and lower plates was used. A plurality ofglass substrates were interposed between the plates which were rotatedin the opposite direction, and two main surfaces of each of the glasssubstrates were polished by the polishing pads which are provided on theplates while a polishing liquid was dropped onto the glass substrates.In this case, a suede-type pad (manufactured by Filwel Co., Ltd.) wasused as the polishing pad in the primary polishing process, andpolishing slurry obtained by adding a ceria-based polishing material(SHOROX manufactured by Tohoku Metal Chemical Co., Ltd.; and graindiameter: 1.0 μm) on the market to water such that the content of ceriawas 0.6 mass % was used as the polishing liquid. A 4-way double-sidedpolishing machine (16B manufactured by Hamai Co., Ltd.) was used as thelapping machine and grinding was performed under the followingconditions while a grinding liquid was supplied at a rate of 8liters/minute: the number of rotations of the plate: 30 rpm; pressingpressure: 110 g/cm²; and the processing time: 40 minutes. The amount ofgrinding of one surface of each glass substrate was about 15 μm.

In the secondary main surface polishing process, a polishing machineincluding a pair of upper and lower plates was used. A plurality ofglass substrates were interposed between the plates which were rotatedin the opposite direction, and two main surfaces of each of the glasssubstrates were polished by the polishing pads provided on the plateswhile a polishing liquid was dropped onto the glass substrates. In thiscase, a suede-type pad (manufactured by Filwel Co., Ltd.) was used asthe polishing pad in the secondary polishing process, and polishingslurry obtained by adding a ceria abrasive-containing solution (averagegrain diameter: 0.5 μm; and SHOROX manufactured by Showa Denko K.K.)including 12 mass % of solid and a silica abrasive-containing solution(average grain diameter: 0.08 μm; and Compol manufactured by FujimiIncorporated) including 40 mass % of solid to water such that thecontent of ceria was 0.6 mass % and the content of silica was 0.2 mass %was used as the polishing liquid. A 4-way double-sided polishing machine(16B manufactured by Hamai Co., Ltd.) was used as the lapping machineand grinding was performed under the following conditions while agrinding liquid was supplied at a rate of 7 liters/minute: the number ofrotations of the plate: 25 rpm; pressing pressure: 110 g/cm²; and theprocessing time: 30 minutes. The amount of grinding of one surface ofeach glass substrate was about 2 μm.

Then, a chemical cleaning process using ultrasonic waves and an anionicsurface-active agent and a cleaning process using pure water wereperformed on the obtained glass substrate. In this way, a magneticrecording medium glass substrate according to Example 1 was obtained.

Comparative Example 1

In Comparative example 1, in the primary main surface lapping process, alapping machine including a pair of upper and lower plates was used. Aplurality of glass substrates were interposed between the plates whichwere rotated in the opposite direction, and two main surfaces of each ofthe glass substrates were ground by the grinding pads provided on theplates while a grinding liquid was dropped onto the glass substrates. Inthis case, a resin bond diamond lapping plate using diamond abrasivegrains with an average grain diameter of 9 μm was used as the grindingpad in the primary lapping process, and water was used as the grindingliquid. The lapping plate used in the primary lapping process includeddiamond grains bonded to the entire surface of a grinding surface by abinder and had a flat surface. In addition, the content of diamondabrasive grains was about 20 vol % and a polyurethane-based resin wasused as the binder.

A 4-way double-sided polishing machine (16B manufactured by Hamai Co.,Ltd.) was used as the lapping machine and grinding was performed underthe following conditions while the grinding liquid was supplied to thepress plates: the number of rotations of the plate: 25 rpm; pressingpressure: 120 g/cm²; and the processing time: 15 minutes. A magneticrecording medium glass substrate was manufactured by the same processesas those in Example 1 except for the above-mentioned processes.

Example 2

In Example 2, in the primary main surface lapping process according toExample 1, a metal bond diamond pad was used as the grinding pad. Thediamond pad was obtained by fixing diamond abrasive grains to asubstrate made of a polyurethane-based resin using electroless nickelplating. In the diamond pad, the outside dimensions of a convex portionwere as follows: the convex portion had a size of 2.6 mm×2.6 mm and aheight of 2 mm; the gap between the convex portions was 1 mm; theaverage diameter of diamond abrasive grains was 9 μm; and the content ofthe diamond abrasive grains in the convex portion was about 20 vol %.Manufacturing conditions other than the above were the same as those inExample 1.

Example 3

In Example 3, in the secondary main surface lapping process according toExample 1, a metal bond diamond pad was used as the grinding pad. Thediamond pad was obtained by fixing diamond abrasive grains to asubstrate made of a polyurethane-based resin using electroless nickelplating. In the diamond pad, the outside dimensions of a convex portionwere as follows: the convex portion had a size of 2.6 mm×2.6 mm and aheight of 2 mm; the gap between the convex portions was 1 mm; theaverage diameter of diamond abrasive grains was 4 μm; and the content ofthe diamond abrasive grains in the convex portion was about 50 vol %.Manufacturing conditions other than the above were the same as those inExample 1.

Then, the surface roughness Ra of each of the magnetic recording mediumglass substrates according to Examples 1 to 3 and Comparative example 1was measured. An atomic force microscope (D3000 manufactured by DigitalInstruments Inc.) was used to measure the surface roughness Ra.

As a result, the surface roughness Ra of the magnetic recording mediumglass substrate according to Example 1 was 0.3 nm, the surface roughnessRa of the magnetic recording medium glass substrate according to Example2 was 0.4 nm, and the surface roughness Ra of the magnetic recordingmedium glass substrate according to Example 3 was 0.6 nm. The surfaceroughness Ra of the magnetic recording medium glass substrate accordingto Comparative example 1 was 0.9 nm.

The waviness of the magnetic recording medium glass substrate accordingto Example 2 was 5% less than that of the magnetic recording mediumglass substrate according to Example 1, and the waviness of the magneticrecording medium glass substrate according to Example 3 is equal to thatof the magnetic recording medium glass substrate according to Example 1,and the waviness of all of the magnetic recording medium glass substrateaccording to Examples 1 to 3 was less than that of the magneticrecording medium glass substrate according to Comparative example 1.

As described above, in Examples 1 to 3, it was possible to manufacture aglass substrate (magnetic recording medium glass substrate) with agreater surface smoothness than that in Comparative example 1.

In Examples 2 and 3, the metal bond diamond pad formed using electrolessnickel plating as the binder was used as the grinding pad for theprimary or secondary lapping process. In the metal bond diamond pad, theholding force of the diamond abrasive grains was improved and it isexpected that the lifespan of the pad will increase. However, in Example2, since the metal bond diamond pad was used in the primary lappingprocess, the holding force of the diamond abrasive grains was improved,but the breaking of the diamond abrasive grains occurred. As a result,the polishing capability of the pad according to Example 2 was slightlyless than that of the pad according to Example 1. In Example 3, themetal bond diamond pad was used in the secondary lapping process.However, since the content of the diamond abrasive grains was large, theholding force of the diamond abrasive grains was not very strong. As aresult, the polishing capability of the pad according to Example 3 wasslightly less than that of the pad according to Example 1. In contrast,in Example 1, the diamond pad using the resin as the binder was used andhad an elastic force stronger than the metal bond diamond pad.Therefore, in Example 1, the glass substrate with a surface roughness Rahigher than that of the glass substrate according to Examples 2 and 3was obtained.

INDUSTRIAL APPLICABILITY

The invention can be applied to manufacture a magnetic recording mediumglass substrate.

REFERENCE SIGNS LIST

10: LAPPING MACHINE

11, 12: PLATE

20A, 20B: DIAMOND PAD

20 a: LAPPING SURFACE

21: CONVEX PORTION

22: BASE MATERIAL

30: LAPPING MACHINE

31: INNER CIRCUMFERENCE WHETSTONE

32: OUTER CIRCUMFERENCE WHETSTONE

40: POLISHING MACHINE

41: INNER CIRCUMFERENCE POLISHING BRUSH

50: POLISHING MACHINE

51: ROTATING SHAFT

52: OUTER CIRCUMFERENCE POLISHING BRUSH

60: POLISHING MACHINE

61, 62: PLATE

71: LOWER PLATE

72: UPPER PLATE

73: CARRIER

74: OPENING

75: CONCAVE PORTION

76: PLANETARY GEAR UNIT

77: SUN GEAR UNIT

78: FIXED GEAR UNIT

W: GLASS SUBSTRATE

X: LAMINATE

S: SPACER

1. A method of manufacturing a magnetic recording medium glass substratecomprising: a step of performing a primary lapping process on a surfaceof the glass substrate except for at least an end surface; and a step ofperforming a secondary lapping process on the surface of the glasssubstrate, wherein the primary lapping process and the secondary lappingprocess use diamond pads in which diamond abrasive grains are fixed by abinder, a plurality of convex portions with a flat top are arranged in atile shape on a lapping surface of the diamond pad, in the diamond padused in the primary lapping process, the average diameter of the diamondabrasive grains is in the range of 4 μm to 12 μm and the content of thediamond abrasive grains in the convex portion is in the range of 5 vol %to 70 vol %, and in the diamond pad used in the secondary lappingprocess, the average diameter of the diamond abrasive grains is in therange of 1 μm to 5 μm and the content of the diamond abrasive grains inthe convex portion is in the range of 5 vol % to 80 vol %.
 2. The methodof manufacturing a magnetic recording medium glass substrate accordingto claim 1, wherein, in the diamond pads used in the primary lappingprocess and the secondary lapping process, the convex portion hasoutside dimensions including a size of 1.5 mm to 5 mm×1.5 mm to 5 mm anda height of 0.2 mm to 3 mm, and a gap between adjacent convex portionsis in the range of 0.5 mm to 3 mm.